The Phase Finder describes this idiom to mean you have had a happy experience that has caused you to flush with pleasure. The expression apparently originated from observation that emtional pleasure causes the same kind of flushing of the face created by physical tickling. I recall reading that tickling can become tortorous if carried too far. The companion expression “tickled to death” is used to descibe someone who could not be happier, but the literal meaning is that someone has been tickled to the point breathing becomes difficult. The term “tickled to death” appeared in the St. Nicholas childrens magazine in 1907, and the term “tickled pink” was used by an Illinois newspaper The Daily Review in 1910. For those who want a scientific explanation, the flush (or a blush) is caused by an emotional response that tells the brain the tiny vessels in the face need more blood.